Carrier wave signaling system



oct- 5 Q C. L. WEIS. JR

CARRIER WAVE SIGNALING SYSTEM Filed March 4 1924 2 Sheets-Sheet 1 Q l. E uk //7 en/0r. Mar/e5 L We/S'J:

Patented Get. 5, 1925.

CHARLES L. JR., OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COMI- PANY, INCORPORATED, 035 NEVI' YORK, IN. Y., A. CORPORATION OF NEW YORK.

OABBIERJVAVE SEGNALING vSYS'IEIVII.

Application filed March 4, 1924. Serial No. 696,788.

This invention relates to carrier wave signaling systems and more particularly to systems of carrier current telephony.

The method of signaling by means of high frequency carrier currents and specifically the art of carrier current telephony is now well understood. It is known that a single pair of conductors may be utilized for the transmission of a plurality of messages by means of a plurality of alternating currents of different frequencies. The individual signals, for example speech waves, are so combined with individual carrier currents or waves that variations of the former are impressed on the latter. This process, known as modulation, may be performed in various ways and by various means now well understood. By modulation of the car rier wave, side band frequencies represented by the sum and difference of the carrier and signal frequencies with or without some of the unmodulated carrier components, are produced.

To obtain signal currents from the re ceived modulated waves, they are demodulated, that is, the transmitted side bands are combined with waves of the respective carrier frequencies. In one method of demodulation which is applied to received modulated waves having no carrier components, it is necessary to combine locally produced waves of the different carrier frequencies with the received modulated waves. In another method, in which the car ier frequency waves are transmitted from the modulator, demodulation involves merely the combination of the received side bands and carrier frequency waves.

ln the process of demodulation, there may result in the output circuit of the demodulator, currents of the carrier frequency together with currents of signal frequencies. Since the frequency of the carrier may be within the range of the frequency of signals, interference may result therefrom. One cha acteristic of this interference may be a distal Jing noise.

it is an object, therefore, of this invention to eliminate interference between the currents of carrier frequency and signal requency in a carrier wave signaling system by providing symmetrical paths of low impedance to a particular component of the carrier current.

rents of carrier frequency in the output circuit of a balanced demodulator to prevent the transmission of this component to a low frequency signal circuit.

A. feature of the invention relates to means for supplying symmetrical paths for given components of current from a pushpull demodulator which means forms part of a low pass filter designed to pass waves of signal frequencies.

Another feature is a low pass filter comprising bridged reactances and having means to vary its selectivity to a different portion of the frequency scale.

This invention may comprise the combination of a filter connected between a space discharge demodulator and a hybrid coil and embodied in the receiving channel of a multiplex carrier current telephone system. The filter comprises bridged condensers in the output circuit of the demodulator which have their adjacent plates connected to the filaments of the demodulator tubes. The filter is designed to pass currents of frequencies below a given value and to greatly attenuate, or substantially eliminate, currents of frequencies above this value. The frequencies which the filter is designed to pass are preferably those within the range of signals, and the frequencies which it is desired to exclude are preferably those above this range.

In the drawings, Fig. 1 shows a portion of the terminal circuit of a carrier current telephone system embodying this invention.

Fig. 2 shows another portion of the terminal circuit which comprises the common transmitting and receiving channels.

Fig. 3 shows a self-oscillating demodulator which may be substituted for the portion of Fig. 1 included within the broken lines.

Like reference characters are used throughout the various figures of the drawings .to indicate similar parts.

Referring to Figs. 1 and 2, a terminal of which comprises a low frequency line LFL, a transmitting channel TC, a receiving channel EU, a common transmitting channel CTC, and a common receiving channel CRC, connected. to a. main line ML. The low frequency line LFL is associated with the transmitting and receiving channels TC and RC, respectively, by means of a balanced transformer or hybrid coil 10. In order that channels TCand RC may be conjugate with respect to each other, i. e. in order'that electrical changes in one channel may notproduce effects in the other chan- I group filter TGF. The common receiving channel CRO comprises a receiving terminal amplifier ETA and a receiving group filter RGF. y

The modulator M in transmitting channel TC, may be of the general type shown in the patent to John E. Carson No. 1,3i3,307, issued June 15, 1920. In accordance With. the

. description. given in that patent, this modur lator is of the balanced type in which the unmodulated carrier oscillations are normal.- ly suppressed. However, when low frequency waves transmitted over line LFL are impressed upon the modulator M, side band waves having an amplitude proportional to that of the low frequency waves and frequencies represented by the sum and diilerence of the carrier and signal frequencies are transmitted therefrom. If any difference exists in the characteristics of the two tubes of the modulator or the respective halves of the circuit there may be transmitted. with the side hands a small amount of the unmodulated component of carrier current. Carrier oscillations are supplied to the modulator M by means ofan oscillator TO com prising a space discharge tube 11 having an input circuit connected between its control electrode and cathode and an output cir cuit connected between its anode and cathode tor supplying carrier oscillations to the V modulator.

A feed back circuit, comprising a resonant circuit 13, a resistance 15, and a condenser 25 in series, joins the input and output circuits, whereby energy is supplied from-the output circuit to the input circuit for maintaining sustained oscillations. Condenser 25 prevents direct current from grid polarizing battery 26 from passing through the feed back circuit and also blocks current leaking fromthe grid of the tube.

In addition to the modulator M and its associated. oscillator TO, transmitting chan nel TC comprises transi'nitting band filter Tol which may be of the general type described in the patent to George A. Camp bell, No. 1,227,113, dated May 22, 1917. The band filter TBF is coupled to the output circuit of the modulator M by means of transformer 16 and comprises series inductive elements 17. It also comprises series capacitive elements 21, together with shunt im-pedances 22-, 23 and The output tern'iinals of transmitting band filter Till are connected to the conductors of the common transmitting channel CTG which extend to the primary winding of transformer 27. This winding has connected the eto an adjustable resistance network 28 by which the amplitude the transmitted waves. may be controlled.

Signal modulated waves from the transmitting channel TC together with modulated Waves from other transmitting channels TC TC etc. are transmit-ted to the transmitting terminal amplifier TTA by means of the transformer 27. The amplifier TTA comprises a. plurality of space discharge tubes 31, 32, 34:, 35 and 36 arranged in puslrpull relation in two stages. The tubes 33, 3e, 35 and 36 comprise the second stage and are arranged in parallel. The secondary winding of trans former 2? has its terminals connected respectively to grids of tubes 31 and 32. The output circuit of these tubes is supplied with energy from a battery 37 through a balanced choke coil 38. Blocking condensers 39 are provided. to prevent the passage of direct current to the input circuit of the tubes of the second stage. The filaments of the tubes of the amplifier are connected in series and are supplied with heating current by aseries-paralle1 connection to battery 40. A grid polarizing battery 43, common to the tubes of the second stage, is connected to a divided resistance 44 for impressing a proper average potential upon the grids of the tubes 33 and 3 1-. Supplementary grid po larizing batteries: 41-8 and 4&9 are connected to the grids of tubes 35 and 36 respectively. to compensate for the difference in potential. of the filaments of the tubes. A common plate battery 45 is connected to the plates of tubes 33, 34, 35 and 36 through res-peotive windings of transformer 46. Condens crs 4'? are arranged to supply low impedance paths in shuntto the battery 45 for alternating currents. Resistances 51 enable the filament heating current to be adjusted. Resistances 52 are provided for purposes of balance.

The common transmitting channel ineludes in addition to the amplifier TTA,

the p the transmitting group filter TGF, which may be of the general type described in the U. S. patent to George A. Campbell, No. 1,227,113, granted May 22, 1917. The filter TGF is connected to the amplifier TTA by transformer 16. his filter comprises shunt impedance elements 53 and 5 1 and series impedance elements 55 and 56. The cuput terminals of transmitting group filter TGF are connected to the main line ML.

The common receiving channel CR-C coinprises the receiving group filter RGF which has its input terminals connectec to the main line ML. This group filter is of the same general type as the transmitting group filter TGF and comprises shunt impedance elements and series impedance elements 59. The output terminals of the filter RGF are connected to the primary winding of transformer 61, through an adjustable resistance network 62 by which the amplitude of received waves may be controlled.

The receiving terminal amplifier ETA is substantially identical with the transmitting terminal amplifier TTA. The input circuit of this amplifier comprises the secondary winding of transformer 61. Tie output circuit of the amplifier ETA is connected to the primary winding of transformer 65. The secondary windings 66 and 67 of this transformer are connected in parallel.

The receiving channel RC wh ch has its input terminals connected to the secondary windings 66 and 67 in common with other receiving channels, comprises the receiving band filter REF, the demodulator DM, and the low pass filter LlPF. The receiving band filter REF may be of the general type d sclosed in patent to George A. Campbell, No. 1,227,113, issued J 22, 1917. This band filter comprises series impedance elements 81 and shunt impedance elements 84;, 85 and 8t and is substantially the same as the transmitting band filter TBF previously described with the exception that it comprises no series capacitive elements. The receiving band filter has its input terminals connected to the secondary windings of transformer and its output terminals connected to the primary wind ng of the transformer 87. The secondary winding of the latter transformer is connected in a balanced arrangement to the input circuits of space discharge demodulator DM.

Demodulator DM comprises space discharge tubes 90 and 91 having grid electrodes connected respectively to individual halves of the secondary winding on the transformer 87, which are in series with the individual halves of secondary winding 92 of transformer 93. The battery 9% is arranged to supply a normal average potential to the grid of tube 91. Battery 9 1 in series with an auxiliary battery 95 supplies a normal average potential to the grid of tube 90. The auxiliary battery 95 is employed to compensate for the difference in potential between the filaments of tubes 90 and 91.

The output circuits of tubes 90 and 91, which are connected in balanced relation between their plate and filament electrodes, are connected to the input terminals of a low pass filter LPF comprising shunt condensers 96 and 97 and series inipedances comprising coils 98 and 99 wound on the magnetic core 101. Addit onal condensers 102 and 103 are arranged to be connected in shunt to the coils 98 and 99 by means of switches 104: and 105, respectively.

The output terminals of low pass filter LPF are connected to the primary winding of balanced transformer 1.0. This winding is divided and its adjacent terminals are con nected by a common lead through. condenser 106 to the midpoint between the filament of tubes 99 and 91. Battery 108 has one terminal connected to the mid-point of the pri mary winding of transformer 10 and its other terminal connected to ground.

The filaments of tubes 90 and 91 are con-- nected in series with the filament of oscillator tube 110 to a battery 111.

For supplying carrier frequency oscillations to the demodulator Did, the oscillator lit) is provided which comprises the space discharge tube 119 and its associated input, output and resonant circuits. -his oscillator is identical with the oscillator TO and will not'be further described.

In the operation of this system low fre quency signal waves are received over line LFL and transmitted through balanced transformer 10 to the transmitting channel TC. Because of the balanced network N connected to this transformer which network simulates the electrical characteristics of the line LFL and its associated apparatus, substantially no energy of signal frequency is produced in the receiving channel RC. Thus this channel is, therefore, substantially conjugate with respect to the channel TC for waves of signal frequency. The signal waves are impressed upon the modulator M by means of transformer 1 and are there comb ned with carrier oscillations from oscillator T0. In a manner now well under-- stood, there are produced high frequency currents modulated in accordance with signals in the outputcircuit of modulator M. The signal modulat d waves are transmitted from the modulator through transformer 16 to transmitting band filter TBF.

This band filter is des gned to transmit a band of frequencies ranging above or below he carrier frequency of the channel by an amount equal to the range of frequencies normally utilized in ordinary telephone transmission and therefore passes a single side hand w thout the unmodulated carrier component of the modulated wave.

Modulated carrier currents produced in transmitting channel T0 are transmitted to high frequency line ML through common transmitting channel OTC. This common transmitting channel serves also as a means for transmitt ng modulated carrier currents to high frequency line ML from other transmitting channels similar to TC indicated diagrammatically by T0 TC etc. The modulated carrier currents in the common transmitting channel are transmitted through rei sistance network 28 and transformer 2? to transmitting terminal amplifier TTA. From the amplifier TTA the amplified signal modulated current is transmitted through transformer 41:6 to the transmitting group filter TGF. This group filter is des gned to transmit a group of frequencies covering a range within which are included all the transmitting channel frequencies and to substantially exclude all others. Current from the transmitting group filter is supplied to main line ML.

Modulated in coming carrier currents of the frequencies assigned to receiving channels RC, RC RC etc, are passed by receiving group filter RGF and transn" ed through transformer 61 to the receiving amplifier RTA. Receiving group filter is designed to transmit the group of frequencies utilized by receiving channels RC, RC etc., and to exclude all others including especially the group of frequencies utilized in the transmitting channels TC, TC etc. The receiving terminal amplifier ETA is designed to amplify efficiently currents within the range of frequencies transmitted by group filter RGF. Currents from this am: plifier are transmitted through transformer 65 to the receiving channels RC, RC etc.

Receiving band filter REF is designed to select the side-band of modulated carrier assigned to the particular channel in which it is connected. The current transmitted through this band filter passes by way of transformer 87 to demodulator DM. it is here combined with oscillations of the carrier frequency produced by oscillator R9. flignal frequency currents are thus produced in the output circuit of the demodulator and are transmitted through low pass filter LPF and balanced transformer 10 to the low frequency line LFL.

There will, in general, be present in the output circuit of demodulator Did a certain amount of current of the carrier frequency. One component of this current is produced by the local oscillator and is applied differentially to the coils 98 and 99 of the low I pass filter. Another component of the carrier may be transmitted with the received side hand because of imperfect balance of the circuits. The latter component is applied cumulatively to the coils 98 and 99 and thus is attenuated by the filter. The former component is not so attenuated. This carrier current may be of frequency sul'liciently low to produce interference with signal waves transmitted to the low frequencyline. In order to prevent the transmission of current of the carrier frequency to the low frequency line LFL the low pass filter lJPF comprising the bridged condensers 96 and 97 is provided. This filter is so designed as to pass signal currents without substantial attenuation but to greatly attenuate and to substantially exclude currents of the side band frequency. Bridged condensers supply low impedance paths to the filaments of tubes 90 and 91 for current of the carrier frequency. By closing switches 10% and 105, condensers 102 and 103 may be connected in shunt to the impedance coils 98 and 99 to adjust the filter to a particular carrier frequency.

The system herein disclosed is designed to provide four two-way channels each comprising a transmitting channel and a receiving channel. The carrier frequencies em-. ployed for transmission in one direction may he for example, 6,666, 10,000, 13,333, 16,666 cycles in the four channels respectively; the frequencies employed in the other direction in the four channels may be 28,383, 526,666, 30,000, cycles respectively. In the channels utilizing the group of higher frequencies the lower side band is selected for tigflllSll'llSSlOIl. In the channels utilizing the group of lower frequencies the upper side band is utilized.

In Fig. 3, a space discharge demodulator embodying this invention is shown which may be substituted for the portion of the circuit shown within the dotted line in Fig. 1. This demodulator comprises space discharge tubes 115 and 116 connected in push-pull relation. The grids of the tubes are connected to individual halves of the divided secondary winding of transformer 87 and the midpoint of these windings is connected in series with a coil 11'? and a battery 120 to the filaments of tubes 115 and 116. The plates and filaments of the tubes connect to a bill anced output circuit including low pass filter LPF and the primary winding of balanced transformer 10. The filter LPF is provided with the bridging condensers 96 and 97. A feedback circuit comprising a resistance 118 in series with a resonant circuit 119 is connected between the mid-points of the plates and filaments. The resonant circuit 119 comprise' a variable condenser 121 in shunt to a coil 122 which is in inductive relation to coil 11? whereby energy may be fed back from the output circuit to the input circuit of the demodulator in order to establish oscillations. An adjustable resistance 123 is connected in shunt to the primary winding of transformer 87 whereby the amplitude of received carrier waves may be adjusted.

liitl Battery 120 is provided to supply the pro per average potential to the grids of tubes 115 and 116.

In this demodulator sustained oscillations are produced by the feed back coupling between input and output circuits and demodulation is produced by combining the energy of these oscillations With received modulated carrier waves impressed upon the space discharge tubes 115 and 116. Thus, the oscillator PUG shown in Fig. 1 may be dispensed with.

Although this invention has been described and illustrated in relation to certain specific circuit arrangements, it will be evident that it is capable of application in numerous other organizations of widely different character. The invention is, therefore, not to be limited to the particular embodiment disclosed but only by the appended claims.

lVhat is claimed is:

1. The combination with a balanced demodulator of a symmetrical input circuit and a symmetrical output circuit, a source of modulated carrier current and a source of unmodulated carrier current connected to said input circuit, and selective means in said output circuit comprising a divided element having its mid-point connected to a common return lead of said demodulator to pass signal currents resulting from demodulation but to greatly attenuate currents of the carrier frequency.

2. The combination Which comprises a space discharge demodulator having an input circuit and a source of modulated carrier current associated therewith, an output circuit, a feed back circuit interlinked with said output and input circuits for producing oscillations of carrier frequency, and means in said output circuit for transmitting currents of signal frequency, said means comprising bridged reactances for supplying symmetrical paths of low impedance for a particular component of current of carrier frequency.

3. In combination, in a carrier current signaling system, a balanced demodulator and a balanced transformer, a filter intermediate to said demodulator and transformer, said filter comprising bridged reactances arranged in balanced relation to said demodulator for supplying 10W impedance paths for a given component of current of the carrier frequency.

1. The combination, in a carrier current telephone system of a circuit for transmitting carrier waves of a particular frequency modulated in accordance with signals, a circuit for receiving modulated carrier wavesof a particular frequency, said receiving circuit comprising a space discharge demodulator comprising discharge tubes in push-pull relation and having input and output circuits, and a filter and hybrid coil connected to the output circuit of said demodulator, said filter comprisingcondensers bridged across respective output circuits of said demodulator tubes and arranged to supply shunt paths for particular components of the carrier frequency.

5. In a Carri current telephone system, a plurality of transmitting channels and a plurality of receiving channels each having predetermined assignment of carrier frequencies, selective means common to the re ceiving channels designed to pass carrier frequencies assigned thereto and to exclude others, selective means common to said receiving channels designed to pass carrier frequencies assigned thereto and to exclude others, each of said receiving channels cornprising a push-pull demodulator, a hybrid coil and a low pass filter intermediate there to, said filter comprising elements bridged across the output circuit of said demodulator and having a common return lead symmetrically connecting to said demodulator.

6. In a carrier current telephone system, a plurality of receiving channels, each connected to a 10W frequency circuit by a hybrid coil and having a predetermined assignment of carrier frequencies, selective means to exclude from respective receiving channels waves of frequency other than those assigned thereto and individual low pass filters for said receiving channels connected intermediate to a respective balanced space discharge demodulator and hybrid coil to prevent the transmission of Waves of the assigned carrier frequency to said coil, said filter comprising bridged reactances in balanced relation to said demodulator for shunting particular components of the currents of the carrier frequency.

7. In combination a common transmission line, a plurality of receiving circuits and a plurality of transmitting circuits linked thereto, a common transmitting channel for said transmitting circuits comprising a push-pull amplifier in a plurality of stages, one of said stages having a plurality of tubes in parallel, a common receiving channel for said receiving circuits also comprising a push-pull amplifier in a plurality of stages, one of said stages having a plurality of tubes in parallel, said common transmitting and receiving channels being intermediate to respective transmitting and receiving circuits and said common transmission line, a balanced transformer connected to individual receiving and transmitting circuits, and means to exclude-therefrom carrier currents, transmitted from a demodulator in the associated receiving circuit said means comprising a filter having bridged condensers connected across an output circuit of said demodulator to shunt particular components of current of carrier frequency.

8. In a carrier Wave signaling system, a space discharge demodulator including space discharge tubes arranged in push-pull relation and having input and output circuits, said input circuit being impressed With Waves of carrier frequency and a filter in said output circuit to greatly attenuate currents of the carrier frequency said filter comprising tWo shunt capacitive elements only and series inductive elements, said shunt elements having respective individual connection to separate output circuits and a common connection to ground.

9. In a selective signaling system, a space discharge demodulator having tubes in push-pull relation impressed with current of carrier frequency, a filter supplied With current from said demodulator for attenuating currents of the carrier frequency and upward said filter comprising means for adjusting the attenuation thereof, and means for supplying symmetrical shunt paths to particular components of current of the carrier frequency from said demodulator.

In Witness whereof, I hereunto subscribe my name this 29th day of February A. D., 19245.

CHARLES L. WVEIS, JR. 

